Power Electronics andDrives (Version 3-2003).Dr. Zainal Salam, UTM-JB32Application examples• For each of the following application, choose thebest power switches and reason out why.– An inverter for the light-rail train (LRT) locomotiveoperating from a DC supply of 750 V. Thelocomotive is rated at 150 kW. The induction motoris to run from standstill up to 200 Hz, with powerswitches frequencies up to 10KHz.– A switch-mode power supply (SMPS) for remotetelecommunication equipment is to be developed.The input voltage is obtained from a photovoltaicarray that produces a maximum output voltage of100 V and a minimum current of 200 A. Theswitching frequency should be higher than 100kHz.– A HVDC transmission system transmitting power of300 MW from one ac system to another ac systemboth operating at 50 Hz, and the DC link voltageoperating at 2.0 kV.Power Electronics andDrives (Version 3-2003).Dr. Zainal Salam, UTM-JB33Power switch losses• Why it is important to consider losses of powerswitches?– to ensure that the system operates reliably underprescribed ambient conditions– so that heat removal mechanism(e.g. heatsink, radiators, coolant) can be specified. lossesin switches affects the system efficiency• Heat sinks and other heat removal systems arecostly and bulky. Can be substantial cost of the totalsystem.• If a power switch is not cooled to its specifiedjunction temperature, the full power capability ofthe switch cannot be realised. Derating of the powerswitch ratings may be necessary.• Main losses:– forward conduction losses,– blocking state losses– switching lossesPower Electronics andDrives (Version 3-2003).Dr. Zainal Salam, UTM-JB34Heat Removal MechanismSCR (stud-type) onair-cooled kitsPower Electronics andDrives (Version 3-2003).Dr. Zainal Salam, UTM-JB35Forward conduction lossIdeal switch:– Zero voltage drop across it during turn-on (Von).– Although the forward current ( Ion) may belarge, the losses on the switch is zero.• Real switch:– Exhibits forward conduction voltage (on state)(between 1-3V, depending on type of switch)during turn on.– Losses is measured by product of volt-dropacross the device Vonwith the current, Ion,averaged over the period.• Major loss at low frequency and DC+Von−IonIdeal switchIon+Von−Real switchPower Electronics andDrives (Version 3-2003).Dr. Zainal Salam, UTM-JB36Blocking state loss• During turn-off, the switch blocks large voltage.• Ideally no current should flow through the switch.But for real switch a small amount of leakagecurrent may flow. This creates turn-off or blockingstate losses• The leakage current during turn-off is normallyvery small, Hence the turn-off losses are usuallyneglected.Power Electronics andDrives (Version 3-2003).Dr. Zainal Salam, UTM-JB37Switching loss• Ideal switch:– During turn-on and turn off, ideal switch requireszero transition time. Voltage and current areswitched instantaneously.– Power loss due to switching is zero• Real switch:– During switching transition, the voltage requires timeto fall and the current requires time to rise.– The switching losses is the product of devicevoltage and current during transition.• Major loss at high frequency operationvitimeIdeal switching profile(turn on)vitimeReal switching profile(turn-on)P=viEnergyPower Electronics andDrives (Version 3-2003).Dr. Zainal Salam, UTM-JB38Snubbers• PCB construction, wire loops creates strayinductance, Ls.• Using KVL,timeVceVcerated+Vin−Ls+Vce−+VL−idtdiLvvdtdidtdiLvvvdtdiLvvvsincesincecescesin+=−=+=+=off) (turning negative is sinceSimple switch at turn offPower Electronics andDrives (Version 3-2003).Dr. Zainal Salam, UTM-JB39RCD Snubbers• The voltage across the switch is bigger than thesupply (for a short moment). This is spike.• The spike may exceed the switch rated blockingvoltage and causes damage due to over-voltage.• A snubber is put across the switch. An example of asnubber is an RCD circuit shown below.• Snubber circuit “smoothened” the transition andmake the switch voltage rise more “slowly”. Ineffect it dampens the high voltage spike to a safevalue.+Vce−LstimeVceVceratedPower Electronics andDrives (Version 3-2003).Dr. Zainal Salam, UTM-JB40Snubbers• In general, snubbers are used for:– turn-on:to minimise large overcurrentsthrough the device at turn-on– turn-off: to minimise large overvoltages acrossthe device during turn-off.– Stress reduction: to shape the device switchingwaveform such that the voltage and currentassociated with the device are not highsimultaneously.• Switches and diodes requires snubbers. However,new generation of IGBT, MOSFET and IGCT donot require it.Power Electronics andDrives (Version 3-2003).Dr. Zainal Salam, UTM-JB41Ideal vs. Practical power switchIdeal switch Practical switchBlock arbitrarily largeforward and reversevoltage with zerocurrent flow when off